I created a new event in Posteo’s calendar, and set the reminder to 5 minutes before this event begins. Then I saved the event values, and re-opened the editor to check that the values I’ve set are still there, and closed the event editor again.
Then on android (Lollipop) I open DAVdroid and chose my posteo account, and click on the synchronizing button. As a result the newly created event is importet into android’s native calendar.
Now I check that event in android’s calendar by tapping on it. What is see, is “Reminders: None” and when tapping on the “editing” button, and scrolling in android’s native event editor down and tapping on “more”, I see “Reminders: On time”